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Aeolian Dust Dispersal Patterns Since the Last Glacial Period in Eastern

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Aeolian Dust Dispersal Patterns Since the Last Glacial Period in Eastern 1 Aeolian dust dispersal patterns since the last glacial period in eastern 2 Central Asia: Insights from a loess-paleosol sequence in the Ili Basin 3 Yue Li1,2,3, Yougui Song1*, Kathryn E. Fitzsimmons2, Hong Chang1, Rustam Orozbaev4,5, Xinxin 4 Li 1,2 5 1 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese 6 Academy of Sciences, Xi’an, 710061, China 7 2 Research Group for Terrestrial Palaeoclimates, Max Planck Institute for Chemistry, Mainz, 55128, 8 Germany 9 3 College of Earth Science, University of Chinese Academy of Sciences, Beijing, 100049, China 10 4 Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences, 11 Urumqi, 830011, China 12 5 Institute of Geology, National Academy of Sciences, Bishkek, 720040, Kyrgyzstan 13 14 * Corresponding author: 15 Dr. Yougui Song 16 E-mail: [email protected] 17 State Key Laboratory of Loess and Quaternary Geology 18 Institute of Earth Environment, Chinese Academy of Sciences 19 No. 97 Yanxiang Road, Yanta, Xi’an 710061, China 20 Tel. 86-29 6233 6216 21 Fax. 86-29 6233 6216 22 1 23 Abstract 24 The extensive loess deposits of the Eurasian mid-latitudes provide important terrestrial 25 archives of Quaternary climatic change. As yet, however, loess records in Central Asia are poorly 26 understood. Here we investigate the grain size and magnetic characteristics of loess from the Nilka 27 (NLK) section in the Ili Basin of eastern Central Asia. Weak pedogenesis suggested by frequency- 28 dependent magnetic susceptibility (χfd%) and magnetic susceptibility (MS) peaks in primary loess 29 suggest that MS is more strongly influenced by allogenetic magnetic minerals than pedogenesis, 30 and may therefore be used to indicate wind strength. This is supported by the close correlation 31 between variations in MS and proportions of the sand-sized fraction. To further explore the temporal 32 variability in dust transport patterns, we identified three grain size end members (EM1, mode size 33 47.5 µm; EM2, 33.6 µm; EM3, 18.9 µm ) which represent distinct aerodynamic environments. EM1 34 and EM2 are inferred to represent grain-size fractions transported from proximal sources in short- 35 term, near-surface suspension during dust outbreaks. EM3 appears to represent a continuous 36 background dust fraction under non-dust storm conditions. Of the three end members, EM1 is most 37 likely the most sensitive recorder of wind strength. We compare our EM1 proportions and mean 38 grain size from NLK with the Jingyuan section in the Chinese loess plateau, and assess these in the 39 context of modern and Holocene climate data, and suggest that the Siberian high-pressure system is 40 the dominant influence on wind dynamics and thus loess deposition in the eastern Ili Basin. Six 41 millennial-scale cooling (Heinrich) events can be identified in the NLK loess records. Our grain- 42 size data support the hypothesis that the Siberian High acts as teleconnection between the climatic 43 systems of the North Atlantic and East Asia in the high northern latitudes, but not for the mid- 44 latitude westerlies. 45 46 Key words: Last glacial, Ili Basin, Central Asia, loess, magnetic susceptibility, grain size, 47 paleoclimate 48 49 1 Introduction 50 Central Eurasia experiences extremely continental climatic conditions, in large part due to its 51 position far from the oceans. Arid Central Asia (ACA), the mid-latitude region spanning the Caspian 52 Sea across to the eastern Tien Shan mountains, is therefore a sensitive recorder of past climate 53 change due to its location in the transitional region between the Asian monsoons (Dettman et al., 54 2001;Cheng et al., 2012), mid-latitude westerlies (Vandenberghe et al., 2006) and North Asian polar 55 front (Machalett et al., 2008). The relative importance and intensity of these major climate 56 subsystems has varied across the latitudinal and longitudinal range of Central Asia through time. 57 Thus identification of the predominant climate regimes in this region, using geological archives, is 58 a crucial precondition for tracing paleoclimatic evolution. 59 One of the most promising potential palaeoenvironmental archives in ACA is its widespread, 60 thick loess deposits. Loess is one of the most important archives of Quaternary climate change 61 (Maher, 2016;Muhs, 2013). In Central Asia, the loess deposits drape the piedmont slopes of the 62 major mountain ranges - the Tian Shan, Alai, Altai and Pamirs - from Xinjiang province in China 63 through Kazakhstan, Kyrgyzstan and Uzbekistan, into Tajikistan. While recent years have witnessed 64 increasing loess-based datasets in the region (Dodonov et al., 2006;Feng et al., 2011;Li et al., 65 2016c;Li et al., 2016b;Machalett et al., 2006;Smalley et al., 2006;Song et al., 2014;Song et al., 66 2015;Song et al., 2012;Yang et al., 2006;Youn et al., 2014;Fitzsimmons et al., 2016), the forcing 2 67 mechanisms and the climatic conditions responsible for loess-paleosol sequences formation are as 68 yet not systematically understood (Fitzsimmons et al., 2016;Machalett et al., 2008;Li et al., 69 2016a;Song et al., 2017b). 70 Evidence for millennial-scale climatic oscillations associated with the Greenland 71 (Dansgaard/Oeschger (D-O) events) (Dansgaard et al., 1993) and cool phases associated with 72 iceberg calving into the North Atlantic (Heinrich (H) events) (Bond et al., 1992) have been found 73 in the form of grain-size variations in loess deposits in Chinese Loess Plateau (CLP) (Sun et al., 74 2012;Porter and An, 1995;Chen et al., 1997b) and Europe (Antoine et al., 2009;Rousseau et al., 75 2007;Zeeden et al., 2016). Data for Central Asian loess do not as yet exist at this resolution (Li et 76 al., 2016b;Song et al., 2017a), despite its strategic location as a likely environmental bridge between 77 the North Atlantic and East Asian Monsoon climatic regions. 78 The Ili Basin of Central Asia hosts thick loess deposits in the strategic central eastern part of 79 ACA (Song et al., 2014). The basin is surrounded to the south and north by the Tian Shan mountain 80 range, widens to the west and drains into endorheic Lake Balkhash (Fig. 1), and provides a 81 conducive situation for loess accumulation. In this paper we present new data on the physical 82 properties of a 20.4 m thick loess deposit at Nilka (NLK) in the eastern Ili Basin. We investigate 83 variations in grain size distributions and magnetic properties in order to investigate likely links with 84 environmental dynamics. 85 2 Physical geography 86 The Ili Basin (78° ~ 85° E and 42° 30ʹ ~ 44° 30ʹ N) straddles southeast Kazakhstan and 87 northwest China. It is an intermontane basin opening westward towards the Ili drains into Lake 88 Balkhash which is in the semi-arid transitional region between the steppe and full deserts of Central 89 Asia. The Northern and Southern Tian Shan form the northern and southern margins of the basin 90 (Fig. 1a). 91 This region has a semi-arid, continental climate, with a strong precipitation gradient dependent 92 on altitude. The altitude of the basin floor is 500 ~ 780 m; the northern Tian Shan Range reaches 93 altitudes of > 4000 m a.s.l. and the southern Tian Shan mountains range between 3000 ~7000 m 94 a.s.l. towards the catchment divide. Mean annual precipitation (MAP) ranges between 200-500 mm 95 on the plains, and mean annual temperature (MAT) ranges from 2.6-10.4°C (Li, 1991;Ye, 1999). 96 The surface vegetation in this region is dominated by Desert Steppe and Steppe and the zonal soils 97 comprise Sierozem, Castonozem and Chernozem. 98 Modern meteorological data (2009 ‒ 2013) show a MAP of 354 mm and a MAT of 7.3°C in 99 Nilka site (data from the China Meteorological Data Network: http://data.cma.cn/). 100 101 Fig. 1 The location of study area and the photo of Nilka (NLK) section. 102 103 3 Materials and methods 104 3.1 Section and sampling 105 The Nilka (NLK) section (83.25°E, 43.76°N, 1253 m a.s.l) is situated on the second terrace of 106 the right bank of the Kashi River, a tributary of the Ili River. The site is located in the eastern Ili 107 Basin of far western China, adjoining the Northern Tian Shan to the north (Fig. 1b). The section has 108 a thickness of 20.4 m and overlies fluvial sands and gravels (Fig.1c). The profile has been exposed 109 by local residents for making bricks, and recently formed the focus of a geochronological study 110 comparing luminescence with radiocarbon methods (Song et al., 2015). According to the dating 3 111 results, the NLK loess started to accumulate since ~ 70 ka. Stratigraphically and geochronologically, 112 the loess package at NLK is equivalent to the L1 loess unit (also known as Malan loess) and S0 113 paleosol unit (known as Holocene Heilu soil) in the CLP (Liu, 1985a). Although largely 114 homogeneous in appearance, two weak paleosols (at 5.0 – 7.0 m and 15.7 – 18.0 m depths) were 115 identified in the section by field observations. We therefore divided the NLK stratigraphy into S0, 116 L1L1, L1S1, L1L2, L1S2 and L1L3 units (Fig. 1c). 117 After cleaning the NLK section to remove dry, weathered sediment, samples were collected at 118 intervals of 2 cm. A total of 1026 bulk samples were prepared for measurements of physical 119 characteristics. Because the optically stimulated luminescence (OSL) dating is more reliable for 120 constructing a loess chronology than bulk sediment AMS 14C dates (Song et al., 2015), this study 121 uses the previously published OSL dating results as the basis for our age model.
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